This invention relates to a disc brake system and, more particularly, to a disc brake system for use on production vehicles.
Currently, production vehicles such as automobiles often have spot-type disc brakes, having a fixed brake disc and a caliper configuration with an inboard piston and cylinder operated by hydraulic fluid. The caliper is bolted to a suspension member either forward or rearward with respect to a vertical plane through a wheel axle and includes a slidable bridge sliding on pins with an outer brake pad on the outboard end of the bridge. Operation of the brake pedal forces the piston outwardly to engage and to slide an inner brake pad along the bridge into frictional braking engagement with the inner side of the fixed disc, which is rigidly fixed to a hub on which the wheel is mounted. A reaction force on the slidable bridge causes it to slide on the pins and force the outer brake pad tightly against the outer side of the fixed brake disc. Deceleration of the clamped disc and its associated hub and its attached wheel thereby decelerates the vehicle. As the piston is forced to slide to engage the brake pads with the fixed disc, an internal O ring seal between the cylinder and the piston is compressed, and energy is stored therein which is released, when the braking fluid pressure is relieved, to slide the piston in the reverse direction to its off brake position in the cylinder.
This conventional disc brake hereinafter called a "conventional, fixed disc brake" because its brake disc is fixed to the wheel hub. In contrast thereto, there is disclosed in the patent literature, such as U.S. Pat. Nos. 4,844,206 and 4,576,255; U.K. patent application 2 184 801; and South African published application 70/5340, a pair of slidable brake discs that slide axially along a wheel hub and utilize a fixed bridge with a fixed brake pad at a distal end of the bridge; and a hydraulic piston that slides the brake discs and slidable brake pads outwardly to bring an outer side of the outer brake disc into braking engagement with the distal, fixed brake pad. The fixed disc brake system is widely used, particularly to brake the front wheels while the slidable brake disc system is not currently in use on production vehicles. In order to be used on vehicles, any brake system must meet a long list of demanding specifications, some of which are explained herein. Until now, slidable brake disc systems appear to be unable to meet the rigorous, demanding criteria to the satisfaction of automobile manufacturers or suppliers. Vehicle manufacturers and brake suppliers undertake the risk of product liability lawsuits or product recalls and, therefore, are reluctant to adopt a new braking system unless it has superior qualities such as improved cost, weight, efficiency, longevity or other qualities relative to the standard fixed disc brake.
The current fixed brake disc systems used on vehicles are quite heavy and a reduction in weight is a desirable goal for the slidable disc brake system. In the fixed brake, the sliding bridge is quite large and heavy as are the bolts to bolt the caliper unit to the suspension member and heavy slide pins are used to support the slidable bridge. The typical brake disc itself is also quite heavy with its bell or hat shape and with its annular rim for engaging the brake pad.
Weight of the fixed brake disc system is detrimental not only to fuel efficiency but also to steering. That is, the brakes represent an unsprung mass on the wheel that must be turned and steered and that also must be supported to withstand high loads including the brake torque and loads due to a wheel going up and down as it travels over uneven road surfaces. The overall size of the fixed, disc brake and its location on the vehicle suspension requires a large space envelope that limits the locking angle and vehicle turning circle, particularly for some types of wheel suspensions.
In addition to size, cost and weight, there are the criteria of efficiency, proficiency and longevity. Brake wear is a longevity problem and a longer brake pad life and brake disc life are desired by vehicle manufacturers which are increasingly providing long term service warranties for their vehicles, as well as for the vehicle owner who ultimately pays for brake replacement in one manner or the other. The brake disc life can be adversely affected by a localized, rubbing contact between the brake pads and the brake pad, particularly at the brake-off position of the braking system. If the brake disc is tilted from a true, vertical plane normal to a horizontal axis through the hub, an increased localized rubbing contact of brake pads on the disc results and is a source of disc thickness variation (DTV), i.e., a different thickness in cross-section of the fixed brake disc at different radial locations from the disc axis. Significant DTV results in vibrations that the driver feels and requires costly brake maintenance to eliminate the vibration problem.
Some fixed disc brake systems have noise problems which are cured to a certain extent by the addition of noise suppressors, which add to the size, weight and cost of the system. Brake systems must be free of rattles and should be free of noise. The current bell or hat shape of the conventional fixed brakes can be noisy because an impact on the disc causes a noise resonance and loud sound due to its bell shape and fixed attachment to the hub. Therefore, it is desirable to eliminate such a noisy shape and fixed mounting of the brake disc to the hub.
In addition to above, there also may occur a "feel" problem where the driver experiences a long or deep pedal depression when operating the brakes. Sometimes the deep pedal is the result of "knock back" of the actuator piston in the cylinder in the piston return direction that displaces hydraulic liquid and shifts the piston deeper in the cylinder. A cornering or bumps in the road may deflect the suspension or slidable brake caliper and knock back the piston and cause the vehicle operator to experience a deep pedal braking operation. Another potential source of "knock back" in the conventional sliding bridge, fixed brake disc system is the result of an initial large deflection of the outer distal end of the slidable bridge at heavy braking loads where the bridge distal end is often deflected 0.006 inch or more. The sliding bridges are already quite heavy and massive to withstand the braking torque and to provide the stiffness for the distal end of the bridge. When the brake pressure is released, the distal bridge end rebounds and can cause knock back of the piston in the cylinder.
Among the demanding temperature tests that vehicle brakes are subjected to is the Auto Motive Sport (AMS) fade test in which the brake temperature is monitored during ten braking stops as fast as possible over a very short time interval. In brief, the AMS vehicle test involves the driver flooring the gas pedal to accelerate vehicle extremely fast to 100 Kph. and then braking as hard as the driver can to stop the vehicle as quickly as it can be braked to a stop. This is quickly iterated for a total of ten rapid accelerations and decelerations with the temperatures of the brake disc being measured over the course of the ten cycles. It is desired to keep the maximum temperature of the brake disc below its "Judder" effect temperature at which severe disc geometry and metallurgical changes occur and deteriorate the brake disc. The Judder effect temperature is usually in the range of 650.degree. to 700.degree. C.; and this AMS test is difficult to pass. In that AMS test of a lightweight automobile having the fixed brake system (against which the slidable disc brake system of this invention is compared), the brake disc temperature exceeded 650.degree. C. and the temperature drop between braking operations was only about 30.degree. C. The test data described herein is from two identical, production model automobiles of B Class front wheel drive. They have a kerb weight of about 1000 Kgs. and a gross vehicle weight of about 1350 Kgs.
Manifestly, the test data for different vehicles may vary substantially from that described herein, which data is given for the purposes of illustration of one embodiment of the invention and is to be construed as a limitation for the invention as defined by the claims attached hereto.
Brake disc temperature also can be monitored to provide an indication of "off-brake" residual torque of the braking system. Even though the vehicle operator is not operating the braking pedal and is steering the vehicle along a straight line path, the brake pads are rubbing against the fixed brake and causing the temperature thereof to rise substantially above ambient. The cornering of a vehicle and sharp turning may also shift the slidable brake caliper into rubbing contact with the fixed brake disc. In a current production vehicle of lightweight and having a fixed brake system, the disc temperature was measured at least 35.degree. C. above ambient when the ambient temperature is between 10.degree. to 20.degree. C. This is a good indication that current fixed brake systems have considerable residual torque at the off-brake condition, and concomitant wear and fuel energy waste. It will be recognized that the bell or hat shape of fixed brake disc has non-uniform cross-sectional thickness at the corners and has different fillets that can result in non-uniform expansion thereby causing an increase in the space envelope of the outer rim of the brake disc and resultant rubbing, thereby producing a high off-brake, residual torque.
In order to test conventional brakes when used in mountainous terrain having descending, steep grades with much cornering of the vehicle, a conventional disc brake was tested over thirty descents of a steep mountain with each descent lasting about twenty-four minutes. These fixed brakes discs on a lightweight, production car experienced temperature of over 600.degree. C. after about 13 minutes of descent; and they reached a maximum temperature of almost 680.degree. C. at the end of the run. Hence, there is a need for a disc brake system that runs cooler in such a test so that it does not potentially cause a Judder effect deterioration of the brake disc in mountainous usage. These fixed disc brakes had high brake fluid temperatures and poor standing soaking curves after trans-mountain runs.
It will be appreciated that in sliding brake disc system that brake disc must slide axially on the hub between an off-brake position, where the residual torque should be low and a braking position where the torque is high, and then return to the off-brake position to reduce the residual torque. The sliding connection between the brake disc and hub must be free to move despite being subjected to corrosive conditions and over a long period of use. The slidable brake disc must not be noisy or squeal under low and high temperature conditions, and it must not wobble or generate dust or produce vibrations that the driver can feel or hear. In the patent literature, such as U.K. patent application 2 184 802 and U.S. Pat. No. 4,576,255, the slidable brake disc systems had spline grooves oversized relative to the size of the disc teeth inserted into the grooves and spring devices where mounted on the hub to push the discs to rotate a driving side, flank of each tooth into mating engagement with a flank of the oversized spline groove. The oversized notches were used to prevent the previously heated and now cooled disc from jamming in the splines. To eliminate "knock back" and chattering, springs were inserted into the spline notches to bias the spline flanks on the disc and hub into engagement with one another. Such designs do not provide a good drive connection between the discs and the splined hub, are costly and apparently allow the discs to wobble relative to the hub at high braking loads.
From the foregoing, it will be seen that there is a need for a better, slidable disc mounting system that is more efficient and that does not generate noise or squeal as the brake discs expand at high temperatures and that does not wobble at high braking loads. Also, there is a need for a twin disc braking system that operates at low residual torque in an off-brake condition to reduce DTV and energy loss.